Received July 28, 1997
Amides of fatty acids are lipid bioregulators formed from long chain
saturated and unsaturated fatty acids via amidation by the
corresponding amines. Ethanolamides of fatty acids are the most
well-studied species of this group; an alternative pathway for their
biosynthesis includes hydrolysis of N-acylated
phosphatidylethanolamines by phospholipase D. Ethanolamides of fatty
acids bind to the cannabinoid receptors of the central nervous system
(CB1) or peripheral tissues (CB2) and can be considered as endogenous
ligands of these receptors. Their pharmacological properties are
similar to that of cannabimimetics. Simple amides of fatty acids are
also endogenous bioregulators acting like sleep-inducing (oleamide) or
angiogenic factors (erucamide). A new group of bioregulators comprise
the amides of fatty acids and biologically active amines
(vanillinamine, dopamine, and serotonin).
KEY WORDS: anandamide, oleamide, erucamide, ethanolamides of
fatty acids, amidohydrolase, cannabinoid receptor,
N-acylphosphatidylethanolamine, serotonin, dopamine

Amide derivatives of fatty acids (FA) are widespread in nature. They are
incorporated into ceramides [1], glycosphingolipids
[2], N-acylated lipids [3], and
bacterial lipoproteins [4]. N-Acylation by a
saturated FA is an important posttranslational modification of proteins
[5].

Amides of FA and primary amines (ethanolamine) attracted attention as
bioregulators for the first time in 1957 when is was demonstrated that
N-palmitoylethanolamine is an anti-inflammatory factor contained in the
lipid fraction of soybean, peanut oil, and eggs yolk [6]. A second birth of interest in FA amides with
primary amines is associated with the discovery of potent
neuromodulatory effects of natural N-arachidonylethanolamide
(anandamide) [7] and oleoylamine (oleamide) [8]. Bioeffector characteristics of these and similar
derivatives and their biosynthesis and metabolism are reviewed in the
present manuscript.

ETHANOLAMIDES OF FATTY ACIDS

Cannabinoid Receptors

The study of pharmacological properties of cannabis resulted in
isolation and characterization of a group of compounds designated
cannabinoids. Cannabinoids are ligands of the specific brain membrane
receptor (central cannabinoid receptor or CB1) coupled to the
Gi-protein [9]. The receptor includes
seven transmembrane domains similar to the transmembrane domains of
rhodopsin [10] and upon solubilization, it is
still able to bind the substrate and interact with G-protein [11]. Human [12] and rat
cannabinoid receptors [13] are 97% homologous
(100% in transmembrane domains). Rat CB1 is detected by several days
post partum [14] and is abundant in the brain,
especially in hypothalamus, cortex, basal nuclei, cerebellum, and
olfactory structures [15, 16]. It is important that the mRNA of CB1 was
detected in the spleen, tonsils, and peripheral blood leukocytes by the
polymerase chain reaction method [17].

Peripheral cannabinoid receptor (CB2) that was also discovered recently
[18] has 44% homology with human brain CB1
protein. Similar to CB1, CB2 is coupled to adenylate cyclase via the
Gi-protein [19, 20]. The receptor mRNA is the most abundant in the
human spleen and tonsils. The level of mRNA is maximal in B cells and
NK cells and is less abundant in monocytes and polymorphonuclear
leukocytes; the lowest level was detected in CD8+ and
CD4+ cells [21]. Cannabinoid-binding
sites were detected in B cells of the spleen, lymph nodes, and Peyer
patches [22] and in mast cells [23].

The exact physiological role of cannabinoid receptors is still unknown.
Considering certain psychotropic effect of cannabinoids, CB1 can
participate in the regulation of motility, memory, emotions, and pain
sensitivity as well as in the regulation of vegetative functions of the
body, whereas CB2 is apparently associated with modulation of
immunocompetent cells.

The presence of cannabinoid receptors indicate that a natural mimetic
agent exists in the organism, and its physiological effects are similar
to that of exogenous cannabinoids. This agent was isolated from porcine
brain [7] and identified as ethanolamide of
arachidonic acid designated anandamide by the authors (from the
Sanskrit "ananda" that means bliss) (Fig. 1). Anandamide competitively inhibited specific
binding of radioactive cannabimimetics with synaptosomal membranes and
induced dose-dependent inhibition of contraction of electrically
stimulated mouse vas deferens; it also exhibited other
pharmacological properties characteristic for psychotropic cannabinoids
[24].

Later, two new ethanolamides of fatty acids were isolated from porcine
brain including dihomo-gamma-linolenoylethanolamine and
docosatetraenoylethanolamine, and they are also ligands of CB1 [25, 26]. It was demonstrated
that two additional ethanolamides of polyunsaturated fatty acids
(docosahexaenoic [27] and eicosatrienoic
(20:3omega9) [28]) are endogenous
cannabinoids.

When cannabinoids interact with cells and tissues expressing CB, they
inhibit stimulated (by forskolin or secretin) adenylate cyclase
activity and opening of N-type calcium channels [29]; these processes are modulated by pertussis toxin
[30] which blocks the dissociation of the
alpha-subunit from the beta,gamma-subunits of the
G-protein. In various cell models, it has been shown that ethanolamides
of polyunsaturated FA also inhibit stimulated activity of adenylate
cyclase [31, 32] and opening
of N-type calcium channels [32, 33] and activate potassium outward current [34]. In CHO and AtT-20 cells expressing CB1 and CB2,
stimulation of these receptors has similar effects on adenylate cyclase
that is reversed by pertussis toxin. It is important that unlike CB1,
stimulation of CB2 had no effect on ion channels [35]. Inhibition of adenylate cyclase was enhanced by
addition of the specific inhibitor of serine amide hydrolases [36] (see below). These effects, similar to exogenous
cannabinoids, were reversed by pertussis toxin [33]. CB-binding characteristics of various
ethanolamides of FA depend on the length and structure of the fatty
acid moiety (see the table). However, the data of different authors or
in different test systems obtained with the very same compound can vary
by several dozens of times. In most cases, the literature constants
correspond to the displacement of radioactive cannabinoids from the CB
preparation by the substances but not to the direct binding of
anandamide and similar compounds to the receptor.

The CB2 peripheral cannabinoid receptor has good affinity for natural
cannabinoids (for example, THC) and synthetic cannabimimetics
(WIN-55,212-2 and CP-55,940) but low affinity to ANA [18] which displaced radiolabeled ligands from their
complex with CB2 ~30-fold less efficiently than with CB1 (see the
table). Synthetic cannabinoids significantly inhibited mouse
anti-dinitrophenol monoclonal IgE antibody-stimulated secretion of
[3H]serotonin by the rat basophilic leukemia RBL-2H3 cells
which expressed CB2 [23]. ANA displaced
[3H]WIN-55,212-2 from the membrane but did not block
serotonin secretion. Addition of ANA (but not arachidonic acid and
ethanolamine) together with WIN-55,212-2 and other active non-lipid
cannabinoids weakened their effects. Hence, ANA is not an agonist but
an antagonist of CB2. N-Palmitoylethanolamine (unlike ANA) exhibited
high activity in [3H]WIN-55,212-2 displacement from the
membrane and in inhibition of IgE-stimulated serotonin secretion. ANA
weakened the effects of PEA and cannabinoids. The data suggested that
PEA and possibly ethanolamides of other saturated fatty acids are
endogenous ligands of CB2 [23]. However, other
data indicate that PEA has low affinity for CB2 in COS-7 cells
expressing the receptor [39]; this could be
explained by the presence of several subtypes of CB2.

2-Arachidonylglycerol, a recently discovered endogenous cannabinoid, is
another candidate for the natural ligand of CB2 [40, 41]; it also binds to CB1
[40] and exhibits specific cannabimimetic
pharmacological characteristics [42]. In the near
future, selective inhibitors of various types of CB receptors would
enable definitive determination of the receptors that bind endogenous
cannabinoids.

Physiological Effect of Ethanolamides of FA

Effects of ethanolamides of polyunsaturated FA. Physiological
effects of ANA and similar compounds are similar to the effects of
cannabinoids although less durable; this can be associated with their
hydrolysis by amide hydrolases. Intraperitoneal administration of ANA
in rats and mice results in catalepsy, analgesia, hypothermia, and loss
of mobility [43, 44]. THC and
ANA enhance the effects of muscimol (GABA receptor agonist) that
induces catalepsy in rats [45]. More detailed
investigation of physiological response of mice administered with ANA
via various routes indicated that intravenous, intraperitoneal and
intraspinal injections of ANA result in loss of sensitivity (by >80%
maximally possible in intravenously and intraspinally injected
animals), significant body temperature decrease (by 2-4°C),
decreased motility, and catalepsy. Maximal effect on motility (decrease
by ~85%) was detected immediately after intravenous and intraperitoneal
administration, whereas the onset developed 10 min after intraspinal
injection. The effects of ANA (1.3-18-fold) in all behavioral tests
were weaker than that of THC. Intraspinal injection of ANA resulted in
loss of sensitivity independently of nor-binaltorphimine, which blocks
the similar effect of THC [46].

ANA and synthetic cannabimimetics inhibit ion channels coupled to the
serotonin 5-HT3 receptor. The effect was not associated with
changes in cAMP [47]. Also, ANA decreased the
level of dopamine in rat striatum, lowered the activity of tyrosine
hydroxylase, and changed the ratio of dopamine D1 and D2 receptors
after intraperitoneal injection [48]. ANA also
inhibited prolactin-luteinizing hormone and growth hormone secretion in
gonadectomized rats [49].

Stimulation of neurons in various regions of the rat brain (with
ionophores, depolarizing agents, and glutamate) induces the synthesis
of N-acylethanolamines and accumulation of their precursors [50, 51]. Glutamate secretion by
the neurons of rat hippocampus is inhibited by activation of the
cannabinoid receptors coupled to a G-protein [52].
Thus, neuron functions can be regulated by N-acylethanolamines via ion
permeability of the membrane, mediator secretion, and intracellular
biochemical processes.

Considering the cardiovascular effects, it is important that ANA
injection causes bradycardia and at first, induces rapid increase with
subsequent prolonged lowering of blood pressure [53]. The hypertensive effect is evidently due to the
direct action of ANA on the vessels and the hypotensive effect is
mediated by ANA-induced inhibition of noradrenaline secretion by nerve
endings in the heart and vessels because this was blocked by specific
antagonist of CB1, SR141716A [54, 55]. ANA-dependent inhibition of noradrenaline
secretion was also observed in rat atrium and vas deferens preparations
[56].

ANA also affects reproductive function. ANA decreased the fertility of
sea urchin spermatocytes by suppressing the acrosomal reaction which
occurs during spermatozoa interaction with the ovarian cell [57]. Later on, it was shown that sea urchin eggs
contain APE and a small amount of ethanolamides of palmitic, stearic,
and arachidonic acids [58]. ANA and its precursors
were detected in the rat testis and biosynthesis of these compounds was
demonstrated in the same tissue [59].

Effects of ethanolamides of saturated fatty acids. The first
detected effect of ethanolamides of saturated FA was the
anti-inflammatory activity of PEA-containing lipid extracts of certain
natural products [6, 60, 61]. At the end of the 1960s, it was shown that PEA
increases nonspecific resistance to various bacterial toxins and
traumatic shock [62, 63] and
decreases the intensity of inflammatory and immune processes [64-66]. PEA enhanced the
resistance of the organism to the toxic effects of anti-cancer drugs
[67] and ethanol [68-70].

It has been shown that ethanolamides of saturated and monoenic FA
influence the functioning of calcium channels by blocking calcium
efflux from cells and organelles under hypoxia [71]. Such characteristics suggested that heart tissue
can be protected from ischemic damage by ethanolamide derivatives of
saturated FA; this was confirmed in guinea pigs. Oleoylethanolamine was
the most potent protector of the heart muscle [72].

Biosynthesis

The first reports on the biosynthesis of ethanolamides of FA in
mammalian tissues appeared long before their role as endogenous ligands
of cannabinoid receptors was demonstrated. They are formed via two
pathways: from free FA and ethanolamine and via hydrolysis of
N-acylphosphatidylethanolamines (Fig. 2). By the
beginning of the 1990s, most of the studies considered the amides of
saturated and monoenic acids. Both synthetic pathways were confirmed
for ethanolamides of most polyunsaturated FA only recently.

The study of phosphatidylethanolamine synthesis with
[14C]ethanolamine revealed that rat liver microsomes can
incorporate the label into a lipid identified as N-acylethanolamine [73]. The synthesis was detected for saturated and
unsaturated FA with 12-24 carbon atom length. Amide derivatives were
not formed if choline and various amino acids were used as amines;
however, phenylamine, N-hydroxyphenylamine, and 1-amino-2-propanol were
N-acylated [73]. Study of other biologically
active amines as substrates of this enzyme system indicated that
palmitic acid forms amides with histamine, tyramine, phenethylamine,
and tryptamine but not with noradrenaline or serotonin [74]. The highest activity of N-acylethanolamine
synthesis was detected in the homogenates of liver, brain, and kidney
especially in the microsomes, although the synthesis was significant in
the presence of mitochondria. Among saturated FA, the best substrates
were myristic, palmitic, and stearic acids; and among unsaturated FA,
these were oleic and linoleic acids [75-79] (Fig. 2).

The synthesis of ethanolamides of polyunsaturated FA is detected in
homogenates of various mammalian tissues [80, 81]. Microsomes and cytosol of various rabbit tissues
can synthesize ethanolamides of saturated and unsaturated FA [81]. ANA synthesis was the most active in brain
microsomes and less active in liver, lung, and kidney microsomes. Among
the cytosolic fractions, significant synthetic activity was detected
only in the brain cytosol. Arachidonic acid was the best substrate
(among all acids) for the membrane and cytosolic preparations from the
brain. The selectivity of the enzyme system for this acid was over
50-fold versus palmitic acid. The synthesis is ATP- and CoA-independent
was similar to the PEA synthesis by rat liver microsomes [75]. The data were confirmed by synthesis of
ethanolamides of various FA in bovine brain membranes. The order of the
kinetic characteristic of the substrates was as follows: arachidonic
> dihomo-gamma-linolenic, 11,14-eicosadienoic > palmitic
> adrenic, docosahexaenoic acids. Synthetic activity was the highest
in hypothalamus, lower in thalamus, striatum, and cortex, and the
lowest in the cerebellum and medulla [82]. The
Michaelis constants for both substrates of the synthetase reaction were
relatively high; their values in the enzyme systems from rabbit [81] and rat brain microsomes [59] were 130 and 135 mM for ethanolamine and 7 and
100 µM for arachidonic acid, respectively. Thus, ANA synthesis
from arachidonic acid and ethanolamine requires significant local
increase in these substances or in their release from precursors, for
example by PLA2 and PLD.

An alternative pathway of FA ethanolamide biosynthesis by hydrolysis of
the corresponding N-acylphosphatidylethanolamines was discovered in dog
heart after myocardial infarction. The damaged myocardial regions
accumulated N-acylethanolamines and their precursor APE; the fraction
of radioactive ethanolamine in these lipids upon incubation with the
heart tissue indicated that the former are synthesized from the latter
[83-85] by a specific
phospholipase. The enzyme catalyzing the APE hydrolysis was also
detected in rat brain [86]. It is active over a
wide pH range (from 4.5 to 8.5) and did not require Ca2+ or
other divalent cations but was inhibited by Zn2+. Salts of
bile acids and other ionic detergents inhibited the enzyme and Triton
X-100 activated the reaction. The enzyme cleaved diacyl and plasmalogen
forms of APE and lyso-APE but had no effect on phosphatidylcholine and
phosphatidylethanolamine. Thus, the substrate specificity and
inhibitor-activator profile indicate that this phospholipase is
different from PLD previously detected in the brain and other mammalian
tissues [87, 88]. Later on,
the presence of enzyme reaction synthesizing ethanolamides of FA from
exogenous APE was confirmed in rat [89] and canine
brain [90].

In the studies performed in 1970s, the formation of ethanolamides of
polyunsaturated FA was not detected and APE found in mammalian tissues
primarily had saturated acids at the N-position, whereas ethanolamides
of polyunsaturated FA longer than 18 atoms were not detected [84, 91, 92]. ANA synthesis from the corresponding APE was
described for the first time in primary culture of rat brain neurons in
1994 [93]. The inhibitors of PLA2
(dimethyleicosadienoic and 3-(4-octadecylbenzoyl)acrylic acids) and PLC
(neomycin) did not affect the ionomycin-stimulated ANA formation as
well as the addition of arachidonic acid to intact neurons incubated
with exogenous PLD, thus indicating that the free acid was not coupled
to ethanolamine in the cells. On the other hand, addition of exogenous
PLD (but not PLA2 and PLC) was essential for anandamide
formation in the unstimulated cells and incubation of synthetic
[3H]N-arachidonylphosphatidylethanolamine and other APE with
the neuron homogenate also resulted in the synthesis of ANA and its
analogs, thus confirming the PLD-dependent formation of ANA [93].

The mechanism of N-acylphosphatidylethanolamine synthesis involves
enzymatic translocation of FA from sn-1-position of diacyl
phospholipids; the amine group is not directly acylated by FA or
acyl-CoA [3]. Trans-acylation is
Ca2+-dependent and is catalyzed by a membrane enzyme of
sarcoplasmic reticulum or mitochondria [3]. The
donors of FA are phosphatidylcholine [94, 95], cardiolipin [96], and
phosphatidylethanolamine; in the latter case, intermolecular and
intramolecular acylation can occur [94]. This
pathway results in the preferential formation of ethanolamides of
saturated FA because unsaturated FA (arachidonic acid in particular)
are rare in the sn-1-position although such phospholipids have
been detected [41]. Thus, the ratio of synthesized
N-acylethanolamines depends on the ratio of the corresponding FA
esterified at the first position of phosphatidylcholine that is the
trans-acylase substrate (Fig. 2).

Metabolism

In mammalian tissues, ethanolamides of FA decompose into fatty acid and
ethanolamine. Amidases catalyzing this reaction were discovered in 1966
in rat liver microsomes [75]. Later it was
demonstrated that the activity of amidase is the highest in the rat
liver; the activity in the brain, testis, kidney, lung, and spleen was
5.5-10-fold lower and activity in the heart was over 60-fold lower than
in the liver [86].

The rat liver enzyme was localized predominantly in microsomes but
mitochondria also exhibited significant activity [79]. Hydrolysis occurred over a wide pH range (5-9)
and activity increased at increasing pH. The enzyme was very specific
towards the bases and FA included into N-acylethanolamines [79]. PEA-hydrolyzing amidase was detected also in
canine brain microsomes [90].

The data on amidase activity in animal tissues considered only
ethanolamides of saturated and monoenic FA. Among polyenic FA, ANA was
the most studied amidase substrate. Its amidase activity was detected
in brain cells and other rat tissues [80] and
activity was the highest in the membrane fraction. The enzyme is not
detected in the heart and muscle. Enzyme activity is maximal at pH 8-9
and at pH below 6 and above 10 it was significantly lower [97]. PMSF was a potent inhibitor of the enzyme
(IC50 = 290 nM [98]). Its presence
increases the activity of radioactively labeled cannabimimetic
displacement by anandamide [99] and enhances the
anandamide-dependent inhibition of electrically stimulated contraction
of guinea pig ileum [100]. Trifluoromethyl
ketones and ethyl alpha-keto esters of FA (especially
arachidonyl trifluoromethyl ketone, IC50 = 1.9 µM [98], and ethyl-2-oxostearate) also efficiently
inhibited ANA hydrolysis, whereas alpha-ketoethanolamides and
ethanolamides were relatively weak inhibitors [101]. Recently, more active inhibitors of amidase
were synthesized including lauryl sulfonyl fluoride (IC50 =
3 nM) [98] and
methyl-arachidonyl-fluoro-phosphonate (IC50 = 2.5 nM) [102].

The amidase was specific to amides of FA, did not hydrolyze the peptide
bond, and had no ceramidase activity [103]. Oleic
acid amide (oleamide, sleep-inducing factor) was a substrate as well
[104, 105]; currently, this
enzyme is designated as a fatty acid amide hydrolase. Studies in
various cells indicate that the substrate specificities of amidases are
variable and depend on tissue type. For example, amidases of rat RBL-1
cells had higher activity towards palmitoylethanolamine
(Km = 20 µM) versus anandamide
(Km = 67 µM), whereas nerve cell amidases
(N18TG2) had the opposite specificity
(Km = 80 and 15 µM for PEA and ANA,
respectively) [106]. It should be pointed out
that the affinity of PEA is higher for peripheral (CB2) receptors and
that of ANA is higher for CB1 brain receptors [23]. Thus, amidase activity is the universal
inactivation mechanism of tissue-specific biologically active
modulators, amidated derivatives of FA (Fig. 2).

Studies of hydrolase and synthetase activities of the enzymes from
porcine brain indicate that both activities are attributable to the
same protein [103]. Local increase in
ethanolamine and arachidonic acid can provide the conditions required
for the reversal of the amidase reaction and synthesis of ANA. However,
the authors did not exclude a specific activation mechanism of the
synthetase reaction of the isolated enzyme that lowers the
Km values [103].

Similar to the corresponding FA, ethanolamides of polyunsaturated FA can
form hydroxy derivatives under the action of lipoxygenases [107, 108]. Various types of
lipoxygenases exhibit variable specificity towards ANA. For example,
12-LO of porcine leukocytes, 15-LO of rabbit reticulocytes, and soybean
15-LO (type I) catalyzed the formation of the corresponding 12- and
15-hydroxy derivatives. However, recombinant human platelet 12-LO had
very weak activity and activity of porcine leukocyte 5-LO was
undetectable [107]. The kinetic parameters of
ethanolamides of arachidonic and eicosapentaenoic acids differed
insignificantly in oxidation by soybean 15-LO (type I) versus the
corresponding acids, thus indicating that they can be involved in
enzymatic reactions of the arachidonic acid cascade [108].

Hydroxy derivatives of ANA had variable interaction with CB. The
15-hydroxy derivative of ANA was still able to inhibit the electrically
stimulated contraction of murine vas deferens but 12-hydroxy
derivative had low activity [107]. Thus, oxidized
derivatives of ANA have their own spectrum of biological activities
that is different from that of ANA. The presence of oxidized
ethanolamides in tissues and their physiological role have yet to be
determined.

PRIMARY AMIDES OF FATTY ACIDS

Simple amides of FA are also bioeffectors. For example, in chicken
chorioallantoid membrane and rat cornea, it was shown that amide of
13-cys-docosenoic acid (erucamide) discovered in the bovine
mesentery is an angiogenic factor [109].
Angiogenic activity was demonstrated by synthetic primary amides of
13-trans-docosenoic acid, 18:0, 20:0, 22:0, 20:4omega6
FA, and to the lesser extent of 16:0, 18:1omega9 FA [109] (Fig. 1). Interestingly,
activities of the corresponding FA were sometimes identical or slightly
lower that of their amide derivatives. Later, angiogenic activity of
erucamide was demonstrated in regenerating skeletal muscle [110]. It was noted in both studies that erucamide
had no proliferative activity in endothelium, muscle, and connective
tissue.

Recently, the amide of cys-9,10-octadecenoic acid (oleamide) was
isolated from the cerebrospinal fluid of sleep-deprived cats; it is the
endogenous factor inducing sleep in mammals [8].
Structural and functional studies indicate that the trans-isomer
of oleamide has similar but significantly weaker effect versus oleamide
and change in the position of the double bond and increasing the length
of the hydrocarbon chain cause dramatic lowering or complete loss of
biological activity [8, 111].
Oleamide is synthesized by the brain cells from oleic acid and ammonium
[112] and is degraded by FA amide hydrolase [113]. Apart from its effect on the central nervous
system, oleamide modulates the function of the immune cells. For
example, it inhibits lymphocyte proliferation [114] and exhibits synergism with ANA; the
trans-isomer of oleamide and its saturated derivative were
inactive. It is important that oleamide does not significantly bind to
CB1 and CB2 [39], and its mechanism of action is
yet to be discovered.

The PLA2 inhibitors as a mixture of simple amides of certain
FA (14:3, 14:2, 18:3, 18:2) were isolated from the culture medium of
actinomycetes (SC0043). Synthetic amides of oleic, arachidonic, and
gamma-linolenic acids also inhibited PLA2 from
porcine pancreas, human synovial fluid, and bee and snake venoms [115] and their activity was comparable to that of
"natural" amides.

AMIDES OF FATTY ACIDS WITH BIOACTIVE AMINES

Studies of the lipid-soluble analogs of capsaicin (vanillinamide of
8-methyl-6-nonenic acid that is the main pungent principle in peppers
of the Capsicum family) in mice indicated that they have
anti-nociceptive and anti-inflammatory activities [116]. Vanillinamide of oleic acid (Fig. 1) was active after oral administration and its
toxicity was lower versus other analogs. The effects can be associated
with selective blockade of sensitive neurons by vanillinamide [116].

Amides of FA and biogenic amines (Fig. 1) have
biological activity as well. Amides of palmitic, stearic, and linoleic
acids and dopamine significantly decreased motility in mice and
potentiated haloperidol-induced catalepsy in rats [117]. Dopamineamides and serotoninamides of
arachidonic and eicosapentaenoic acids protect early sea urchin embryos
from the cytotoxic effects of dopamine and serotonin antagonists and
can be agonists of dopamine and serotonin receptors [118]. Amides of arachidonic acid with dopamine,
histamine, and serotonin inhibit human platelet aggregation induced by
adrenaline or arachidonic acid. The anti-aggregatory effect of these
amides can be associated with their action on arachidonic
acid-initiated biochemical processes in platelets [118].

Amides of oleic, linoleic, and linolenic acids and dopamine are
inhibitors of 5-LO from RBL-1 cells. Linoleyldopamine had the highest
activity, which was ~4-fold higher than that of the specific 5-LO
inhibitor AA-861 [119]; serotoninamide of
arachidonic acid was an irreversible inhibitor of 15-LO from soybean
[120].

Amides and ethanolamides of saturated and unsaturated FA participate in
bioregulation as endogenous bioeffector lipids; this was discovered and
confirmed by recent studies. These relatively simple compounds can be
highly efficient neuromodulators (ANA, oleamide) and regulators of
peripheral organs and tissues (ANA, PEA, and erucamide). High
biological activity, receptor-mediated mechanism (demonstrated for
ethanolamides of FA) and the existence of systems synthesizing and
degrading the amides allow them to be considered as the ancestors of a
new class of bioregulators. This class apparently should include amides
of FA and biologically active amines (serotonin, dopamine, etc.) the
presence of which in intact cells is not yet demonstrated but whose
high and specific biological activity indicates that they are plausible
bioeffectors. Bioactive amides of FA are tightly associated with other
lipid bioregulators. For example, formation of ANA and PEA during
PLD-dependent hydrolysis of APE is associated with the generation of
phosphatidic acid that is an important messenger of the cell regulation
by itself (for reviews see [121]). Anandamide
(and THC) initiate release of arachidonic acid via activation of
cytoplasmic arachidonate-specific PLA2 that is associated
with stimulation of eucosanoid synthesis [122].
On the other hand, FA amides can inhibit PLA2 by decreasing
the content of polyenic FA and lysolecithin. ANA and other
polyunsaturated ethanolamides can participate in oxidative metabolism
similar to the corresponding free acids and potent inhibition of
lipoxygenase by their amides with dopamine and serotonin is another
link between oxylipins1 and bioactive amides of FA. Thus,
bioactive amides of FA are organic components of the family of
bioeffector lipids.

This work was supported in part by the Russian Foundation for Basic
Research (grant No. 96-04-49191).

1 Oxylipins (prostaglandins, leukotrienes, etc.) are the
oxidized metabolites of arachidonic and other polyenic FA formed by
action of an enzyme at least on one stage of their generation.